You’ve got to admire Tom Steyer’s sense of drama. The hedge fund billionaire from California, who has lately turned his attention and fortune to politics, wrote an open letter to Democratic Senate candidate Stephen Lynch on Monday, worded gleefully, for maximum attention.

Reverse your support of the Keystone XL oil pipeline, it read, or issue binding proof that the crude oil it carries will stay in the United States. Do it by “high noon” on Friday, Steyer warned, or he will spend money to defeat Lynch in the primary against Keystone opponent Ed Markey.

The Wild West theatrics — and the ultimatum language — make the letter entertaining and easy to dismiss. “My reaction is, does the money make you that arrogant or were you arrogant anyway?” said Scott Ferson, a Lynch spokesman, who says the congressman is not about to change his stance.

But lurking behind Steyer’s swagger is an interesting idea: that you could use money in politics not just as speech — in the form of ugly TV ads — but as leverage. Even blackmail.

And Steyer’s letter represents an intriguing new tactic for the environmental movement, which has struggled for attention from rank-and-file voters, and thinks big money might be the answer.

Steyer has money, and a willingness to use it. He’s hired Chris Lehane, a hardball public relations guru who worked in the Clinton administration. In California last year, he spent $30 million on a ballot initiative called Proposition 39, which proposed to close $1 billion in corporate tax loopholes. The measure passed with 61 percent of the vote and very little public opposition.

That was partly because Steyer used his money as leverage. He sent an open letter to some major corporations, threatening to run TV ads against them, suggesting that the public relations disaster wouldn’t be worth the fight. The companies backed down.

Now, Massachusetts isn’t California, and a ballot initiative isn’t a multi-issue Senate race. Also, Lynch and Markey signed a “people’s pledge,” which ostensibly means that any money Steyer pours into the primary race can’t go to TV ads or direct mail.

Lehane told me it would go toward guerrilla marketing, opposition research, and a robust field effort and get-out-the-vote drive. It would focus on college campuses, where environmentalist passion runs strong. He won’t give a dollar figure, but said Steyer sees this Senate race as “the current front line of the larger climate change fight.”

So does Craig Altemose, who deserves some credit for Steyer’s involvement. A 27-year-old environmental activist with Harvard degrees in law and public policy, he got arrested opposing Keystone in front of the White House in 2011. He’s been working on climate change for seven years, and fuming about the stalled progress, which he blames on big money from the fossil fuel industry.

Last month, he saw Steyer speak at an anti-Keystone rally in Washington, D.C., and had what he calls a “shot in the dark” idea. He gathered four like-minded millennials, two college students and a high-school student, and drafted a letter to Steyer, asking him to help.

“We are hesitant to suggest that more money in the system is the solution,” they wrote, but this is “a crisis moment.”

Steyer agreed within days. Maybe it wasn’t a difficult sell. Focusing on Lynch seems as good a way as any to draw attention to environmentalists’ argument against Keystone: That the pipeline, hailed as a job creator, would yield few permanent jobs, and wouldn’t make America more energy-independent, since the oil would be exported.

Will Steyer call attention to those issues, or will he become the issue himself? Already, the sniping has begun: A Lynch aide pointed out that Steyer’s hedge fund owns millions of dollars in oil company stock. Lehane said Steyer stepped down from the fund last year and is now spending half of his fortune on public causes, including the environment.

How much his money will help, in this case, will depend on how much the environment can rise to the top of a long list of voter priorities. It will probably depend on young voters, who play a key role in a growing movement to tackle climate change, but who aren’t exactly known for voting in special elections.

Can they make electoral good on their passion? After all, money talks. It often swaggers. But voting wins elections.